1. Field of the Invention
The present invention relates to a method for producing silicon single crystals from which silicon wafers can be prepared. More particularly, the present invention relates to a method for producing silicon single crystals which inhibits the formation of dislocations caused by thermal stress and can grow dislocation-free portions in a high yield, and also relates to silicon single crystals produced thereby.
2. Background Art
The Czochralski (CZ) method is a commonly known technique for growing silicon single crystals to be used in preparing silicon wafers. One existing approach for efficiently producing silicon single crystals of the desired quality by the CZ method involves controlling the temperature of the silicon ingot during growth. For example, Patent Document 1 discloses a technique for rapidly cooling the solid-liquid interface or the vicinity thereof, on a silicon ingot being pulled, thereby increasing the maximum pull rate.
However, in the art disclosed in Patent Document 1, dislocations induced by a thermal stress readily arise due to cooling of the silicon ingot, resulting in a poor productivity and a low yield.
Generally, when dislocations have formed in a silicon single crystal during pulling, the already pulled ingot is melted and re-pulled in order to grow a silicon single crystal having a long dislocation-free portion.
However, in the case in which the pulling and melting of a silicon single crystal is repeated, because pulling takes a long time, the productivity decreases. Moreover, if the attempt to grow a dislocation-free crystal is ultimately abandoned and crystal growth is stopped, a large amount of silicon melt ends up remaining in the crucible. Not only is this a waste of the starting material, there is also a risk that volumetric expansion when the remaining silicon melt solidifies will damage the crucible and heater. For this reason, when dislocations have arisen numerous times in a silicon single crystal and the pulled ingot has been melted and re-pulled, if there remains little hope that a silicon ingot having a long dislocation-free portion can be pulled, the practice until now has been to go ahead and pull the ingot regardless of the presence of dislocations.
Furthermore, because a silicon single crystal in which such dislocations have arisen many times will have large crystal grain slippage due to thermal stresses during growth, after the pulled silicon single crystal has been cooled to room temperature, large residual stresses will arise due to such crystal grain slippage. As a result, the slightest impact when a pulled silicon ingot is removed from the furnace or when a silicon ingot that has been removed from the furnace is transported can easily create stresses that exceed the yield stress of the silicon, causing the crystal to crack.
(Patent Document 1) Japanese Patent Application, First Publication No. H11-199385